1) Field of the Invention
This invention relates to the field of data communications systems and in particular to synchronizing received data signals within data communications systems.
2) Background of the Invention
The problem of synchronizing received signals in data communications systems is well known in the art. For example, in the case of video communications one commonly known result of this problem is difficulty in maintaining synchronization between the audio and video portions of the recorded signals over long periods of time. It is common to record the video signal nominally at thirty hertz prior to transmission in these systems. The clock of the recording device may be very precise and the signal may be recorded substantially correctly prior to transmission. At some later time a receiving device with a different clock may receive the recorded and transmitted signal. The receiving device may also have a clock which is quite accurate, also nominally thirty hertz. However, if the two clocks are even slightly different, as they often are in practice, synchronization problems may result. It will be understood that a difference on the order of one part in several million between the two clocks may be enough to cause loss of synchronization.
Loss of synchronization in this type of system may occur in different ways. As an example consider first the case in which the clock of the receiving device is slightly slower than the clock of the transmitting device. Typically in these systems the receiver buffer size is known at the transmitting end during transmission of the signals. The size of the receiver buffer determines how much data the receiver can store without overflowing. The receiver buffer is emptied by the receiver as it uses recorded data at a rate depending on its own internal clock. When the transmitter clock is slightly faster than the receiver clock there will be a gradual buildup of data stored in the receiver buffer.
At some point, if the receiver clock continues to run slower, the receiver buffer overflows. This causes an unacceptable loss of data. For example, the receiver may eventually lose a frame of video data. This may cause a loss of audio and video synchronization. Moreover, the loss of synchronization may be cumulative.
The converse problem occurs when the receiver clock is slightly faster than the transmitter clock. A conventional way to start the transmission process is for the transmitter to transmit some data to the receiver and partially fill the receiver buffer. The receiver then provides a signal to the transmitter indicating that the process is beginning. When the receiver clock is slightly faster than the transmitter clock, the receiver empties the buffer faster than the transmitter fills it. This causes the receiver buffer to be empty at some point. At this point the receiver may unsuccessfully attempt to access data from the empty buffer for display and be unable to display the next frame of video. Under these circumstances the receiver executes some predetermined strategy such as repeating a previous frame. This again may lead to loss of synchronization between audio bit stream and the video bit stream.
A well known type of transmitter/receiver data communications system wherein these types of synchronization problems may occur is encoder/decoder systems. In some of these prior art encoder/decoder systems separate bit streams representing video and audio are encoded in sections called units. These units may be arranged more or less independently to allow editing of the encoded bit streams. Thus, the units may be marked in some way for an editing process and they may be deleted from one or more of the bit streams, rearranged, or added to the bit streams. In a manner similar to that described with respect to lack of synchronization of transmitter and receiver clocks, altering the time duration of portions of the transmitted bit streams by editing in this manner may also lead to loss of synchronization between the two bit streams.
Conventional encoder/decoder systems which permit editing may have predetermined limitations imposed upon them. For example some limitations may be imposed by standardization specifications. These imposed limitations may include the buffer size of the decoder and the constant rate at which data is transmitted from the encoder to the decoder. Under these conditions, it is known by the encoder at the time of encoding of a bit stream what the size of the decoder buffer within the receiving device is. If the clocks of the encoder and the decoder are perfectly synchronized, the decoder buffer may never overflow or underflow and synchronization between the bit streams may be maintained indefinitely. However, when editing is performed on one of the bit streams, synchronization problems may be introduced into the system. For example, if a few units are removed from the audio bit stream it may get ahead of the video bit stream.